Reuseable frogs. Free frogspawn forever!

I find this disturbing, I don't know why. It's on the subject of pregnancy testing.

Selmar Aschheim and Bernhard Zondek introduced testing based on the presence of hCG in 1928. In their test an infantile female mouse was injected subcutaneously with urine of the person to be tested, and some time later the mouse was killed and dissected. Presence of ovulation indicated that the urine contained hCG and meant that the person was pregnant. A similar test was developed using immature rabbits, the rabbit test. Here, too, it was necessary to kill the animal to check its ovaries. An improvement arrived with the frog test that was still used in the 1950s. A female frog was injected with serum or urine of the patient. If the frog produced eggs within the next 24 hours, the test was positive. In the frog test, the animal remained alive, and could be used again.

So, the chemical from the human forced an animal to ovulate? That's just...gross. Although I am cracking up at the idea of reuseable frogs, even if it does seem absolutely unbelievable that it was still a method used into the 1950s.

Grossness aside (incidentally, I also find it very cruel that the animals were randomly dissected. Wouldn't it have been more humane to just wait and see if you were up the duff?), I still don't understand. Why does a chemical from a human force ovulation in an animal? Especially weird that it works on amphibians - I suppose other mammals make a little bit of sense at least. Enlighten me, science people?

Edit: One other thing. If the mice were injected and then killed some time later...well, don't female mice ovulate anyway? I don't know. Don't most mammals? So how did they know that ovulation was prompted by the injection of human urine? Or am I displaying my extrodinary biological stupidity here?